Most present stylus, or "pen," input systems for a computer consist of a stylus which either produces or detects an electrical signal. The stylus is placed against a piece of some signal medium, typically with a large surface area, which performs the conjugate function. For example, a piece of specially prepared glass might cover a computer display, or be incorporated as a computer display screen. Either case may be referred to as a "display screen." Whenever a stylus approaches or touches the display screen, the stylus may measure a signal which varies across the glass surface. Alternately, the stylus may emit a signal which is measured at the perimeter of the glass. In either case, the position of the stylus can be calculated. The system consists of a signal source, a signal detector, and an interface to a computer. For example, a square wave may be generated by the stylus or screen, and the receiving screen or stylus detector converts the serial data into digital words which indicate position. The sending apparatus, and the receiving apparatus that belongs to it, must share a common time base for the encoded data to be correctly recognized.
Various display systems are known which incorporate the capability for printing or writing onto a display screen, with a detector for sensing and encoding the location of the input device on the display screen and accurately determining input coordinate positions. Rebane et al., U.S. Pat. No. 3,857,022, acoustically locates the position of a stylus tapped against a screen. Whetstone, U.S. Pat. No. 4,514,688, determines the distance of a pointer position from a predetermined reference point by measuring magnetostrictively induced strain waves. More et al., U.S. Pat. No. 4,839,634 describes a portable electro-optic device preferably comprising a flat panel display, pen-sensing surface, and a pen, with means provided for sensing and encoding the position of the pen on the display surface.
Along with location sensing, positional accuracy is obtained by using methods for correcting for distortions and abnormalities in the regular array or the screen surface. Kable, U.S. Pat. No. 4,678,869 describes an apparatus, system and method which employ a surface which is excited with an a.c. source at terminal locations disposed at the edges of the surface. Reducing the area of grid elements at crossover positions and intermediate grounding terminals minimizes disruptive potential caused by capacitive coupling. A correction look-up table is developed as part of the manufacture of the apparatus to provide a regularly incremented sequence of address values developed from a signal domain computation. Nakamura et al., U.S. Pat. No. 4,650,926 provides correction for graphic surface non-linearities in an electrographic system wherein output readings are taken along a physical domain rectangular grid array, and these output readings are adjusted in their signal domain to establish a regularly incremented sequence of address values.
Some systems, such as described in Stefik et al., U.S. Pat. No. 4,814,552, may incorporate multiple input styli for a single computer by supplying identifying information in the encoded bitstream. An acoustic signal from each stylus is designed to be easy to convert to a location, and each contains a distinctive identification code so that the system can determine which stylus is the signal source.
One aspect that all of the above-mentioned systems have in common is that the device or devices supplying input signals and the device receiving the signals must share a common time base in order for the input signal to be correctly recognized.
A problem arises, however, in a system incorporating a family of multiple connected computers, each with an individual input device. In a system in which there are many computers, a user of one computer may wish to indicate an input action on a second computer. In an existing system, an input action could only be indicated on a computer using the device that is connected with that specific computer.
Ideally, a user using a first computer would like to use the stylus belonging to that first computer to indicate an input action on any of the family of computers in the multiple computer system. For example, a user of a portable computer with a pen input may walk up to a display for a second user's computer. The first user would like to indicate an input location on the second user's display, and points to the location on the second display with the pen from the first computer. A sensing input device for the first computer must be able to tell which computer display it is touching and approximate location coordinates for the position it is pointing to.
This problem can be solved by providing the input system with means to calculate the position of a stylus with respect to a computer display surface that may be independent of the computer to which the stylus is connected.
In the system of the present invention, all pens in a system are able to determine which computer they are writing on and an input position relative to that input computer, regardless of whether they belong to that computer, by determining an identifying "signature" of the computer. Each stylus is able to communicate with the software entity which can correctly interpret the measurement by using a computer network. Each stylus must be able to participate in the measurement of its location on any computer in the system, without sharing a time base with that computer. The present invention provides a method for the input device to discern useful information concerning position even though that device belongs to another computer.
The present invention additionally provides an input signal to the system which contains information about the input device source allowing the computer being acted upon the ability to distinguish what computer device is providing the input, so that computer can determine access rights and other information relating to the input device. Both frequency and time domain calculations may be used to recover stylus position and signal source information from the stylus detection measurements.